1. Field of the Invention
The present invention relates to a wireless communication apparatus and a wireless communication method in which an OFDM (Orthogonal Frequency-Division Multiplexing) modulation method is applied to a MIMO (Multi-Input Multi-Output) communication.
2. Description of the Related Art
Wireless networks are attracting attention as systems for freeing users from wiring in known wired communication methods. The standards established on a wireless network includes IEEE (The Institute of Electrical and Electronics Engineers) 802.11, etc.
For example, IEEE802.11a/g adopts an OFDM (Orthogonal Frequency-Division Multiplexing) modulation method, which is one of the multi-carrier methods, as a standard of a wireless LAN. In the OFDM modulation method, transmission data is distributed into a plurality of carriers having frequencies orthogonal to each other, and then is transmitted. Accordingly, the band of each carrier becomes a narrow band, and thus the transmission has very high efficiency in the frequency utilization and is resistant to frequency-selective fading hindrances.
The IEEE802.11a/g standards support a modulation method achieving a communication speed of 54 Mbps at the maximum. However, as for a communication speed, a wireless standard enabling a still higher bit rate is demanded. For example, in IEEE802.11n, which is an extended standard of IEEE802.11a/g, next generation wireless LAN technology is designed in order to develop a high-speed wireless LAN standard exceeding an effective throughput of 100 MBPS.
IEEE802.11n adopts an OFDM_MIMO method using OFDM for primary modulation. MIMO (Multi-Input Multi-Output) communication is a technique for achieving high-speed wireless communication by providing with a plurality of antenna elements in both a transmitter and a receiver using a plurality of space-multiplexed spatial streams. The transmitter distributes and sends out transmission data into a plurality of streams using a plurality of antennas, and the receiver performs signal processing using channel characteristics on the space-multiplexed signal received by a plurality of the antennas. Thus, it is possible to divide space, and to extract a signal for each stream without cross talk (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-44051). MIMO communication makes it possible to enlarge the amount of transmission in accordance with the number of antennas, and to achieve an increase in the communication speed without increasing frequency band.
Also, IEEE802.11n is different from IEEE802.11a/g in a transmission method (Modulation and Coding Scheme: MCS) such as a modulation method, a coding method, etc., and achieves high-throughput (HT) transmission. At the same time, it is necessary to allow the coexistence with a communication terminal (in the following, also referred to as a “legacy terminal”) which executes an operation mode (in the following, also referred to as a “legacy mode”) conforming to the known IEEE802.11a/g. Thus, IEEE802.11n defines “Mixed Mode (MM)” as an operation mode for ensuring the compatibility with IEEE802.11a/g. Specifically, the beginning PHY header of a packet (MIMO signal) includes (1) a preamble (in the following, referred to as a “legacy preamble”) having the same format as IEEE802.11a/g, and, subsequently to this, (2) a preamble (in the following, referred to as an “HT preamble”) having the format specific to IEEE802.11n. Thus, it is possible for a communication terminal conforming to the IEEE802.11a/g standards to send and receive the packets.
FIG. 5 illustrates the format of a packet (in the following, referred to also as a “legacy packet”) based on IEEE802.11a/g. Note that 1 OFDM symbol is assumed to be 4 microseconds (the same in the following). The header portion includes, as a legacy preamble, L-STF (Legacy Short Training Field) including a known OFDM symbol for packet detection, L-LTF (Legacy Long Training Field) including a known OFDM symbol for obtaining synchronization and equalization, and L-SIG (Legacy SIGNAL Field) describing a transmission rate, a data length (note that a value for spoofing a legacy terminal is written in the mixed mode), etc. A payload (data) is transmitted subsequently to this.
FIG. 6 illustrates the format of a packet (in the following, referred to also as an “MM packet”) in the mixed mode being studied by IEEE802.11n. This header portion includes L-STF, L-LTF, and L-SIG, which are completely the same as the legacy preamble, and subsequently includes an HT preamble in an HT format and a payload (data). The MM packet includes a portion corresponding to the PHY payload in the legacy packet in an HT-format. The HT format includes HT preambles and PHY payloads recursively.
The HT preamble includes HT-SIG, HT-STF, and HT-LTF. HT-SIG includes description of the information necessary to interpret the HT format, such as MCS to be applied to the PHY payload (PSDU), the data length of the payload. Also, HT-STF includes a training symbol for improving AGC (Automatic Gain Control) in a MIMO system. Also, HT-LTF includes a training symbol for performing channel estimation at the receiver.
In this regard, in the case of the MIMO communication using two transmission branches or more, it is necessary for the receiver to obtain a channel matrix for space dividing the received signal by channel estimating for each transmission and receiving antenna. Thus, the transmitter transmits HT-LTF in time division from each transmission antenna (refer to FIG. 7). The number of the HT-LTF training symbols is not less than the number of spatial streams.
The legacy preamble in the MM packet has the completely same format as the preamble of the legacy packet, and is transmitted by a transmission method allowing a legacy terminal to decode it. In contrast, the HT format portion subsequent to the HT preamble is transmitted by a transmission method not supported by a legacy terminal. A legacy terminal decodes HT-SIG in the legacy preamble of an MM packet to read that the packet is not for that station itself and the data-length information. Thus, the legacy terminal can set NAV (Network Allocation Vector) only during the correct period to avoid collision. As a result, it is possible for the MM packet to achieve the compatibility with legacy terminals.
Also, the MIMO communication has a problem in that when the same or similar signal is transmitted through a different spatial stream, an unintended beam might be formed. Thus, IEEE802.11n is studying a method in which the transmitter transmits a signal from each transmission antenna with a time difference (Cyclic Shift or CDD (Cyclic Delay Diversity)) (for example, refer to EWC (Enhanced Wireless Consortium) PHY Specification). By this means, the cyclic-shift values among transmission antennas are defined for the legacy preamble portion of an MM packet and an HT-format portion, respectively. For example, when performing the MIMO communication having two spatial streams, in the legacy portion, the second spatial stream becomes a time-difference signal having a delay time of −200 nanoseconds with respect to the first spatial stream.
TABLE 1TCSiTX values for the legacy portion of the packetcyclic shiftcyclic shiftcyclic shiftcyclic shiftNumber of Txfor Tx chainfor Tx chainfor Tx chainfor Tx chainChains123410 ns———20 ns−200 ns——30 ns−100 ns−200 ns—40 ns −50 ns−100 ns−150 ns
As already described, the legacy preamble of an MM packet has the completely same format as the preamble of a legacy packet. However, if CDD is applied, both packets have a difference in whether the cyclic-shift signal is added. When configuring a communication apparatus supporting both IEEE802.11a/g and IEEE802.11n (in the following, also referred to as an “MM terminal”), it is necessary to receive both of a legacy packet and an MM packet correctly. However, if the signal is subjected to a cyclic shift, a problem arises in the timing of which synchronization is obtained by the legacy preamble (specifically, the L-LTF field).
The receiver normally obtains the cross-correlation between an L-LTF receive signal and a known training symbol on the basis of a peak of the auto-correlation in the L-STF portion in the received legacy preamble, and then obtains synchronization on the basis of the peak position thereof. However, if the cyclic-shift signal is not added, the correlation appears somewhere behind the reference (refer to FIG. 8). In contrast, if the cyclic-shift signal with the transmission timing shifted ahead is added, the correlation primarily appears ahead of the reference (refer to FIG. 9). For example, in a system using two spatial streams, if a delay is small, two peaks appear at the original timing and at the timing by the cyclic-shift signal because of the cross-correlation. However, if a delay becomes large, the peak declines and the delay spread appears widely ahead.
Accordingly, as a result of the addition of the cyclic-shift signal, the synchronization might be obtained a few symbols ahead of the original synchronization timing. Thus, the FFT window for the OFDM modulation is shifted ahead. For example, in the case of 20 MHz, a shift of 200 nanoseconds corresponds to 4 samples.
Even when the FFT window is shifted a little ahead in this manner, in the normal case, interference between symbols caused by the preceding symbol does not occur by a guard interval. However, in the case of a propagation channel having a large delay, interference between symbols occurs because of being out of synchronization, and thus an error might occur.
In particular, when the modulation level of 16 QAM, 64 QAM, or higher is used in the HT-LTF and the HT-DATA portion, if interference between symbols occurs, a floor phenomenon, in which an error rate is not improved in spite of an increase in the SN ratio, occurs. In order to avoid such interference between symbols as much as possible, if the FFT window is shifted behind to the utmost, interference from the succeeding symbol might occur on the other way around.
Also, a channel is subjected to phase rotation with time. Thus, if channel estimation is conducted on the HT-LTF field symbol at an incorrect timing, the timing error influences as a phase error, and thus a high-precision channel matrix is difficult to be obtained. There is a problem in that if the received signal is subject to a MIMO synthesis using a low-precision channel matrix, cross talk occurs between the streams, and thus it is difficult to correctly separate the signal into the original spatial spaces.
If a communication terminal is exclusively used for IEEE802.11n, it is sufficient to receive a packet having only an HT format. Thus, the above-described problem is solved by shifting the obtained timing behind by the amount of the known cyclic shift. However, in the case of an MM terminal, the above-described problem is inevitable, because the synchronization timing is obtained in the legacy preamble in a state in which whether the cyclic-shift signal is added is not yet determined.